16 Hugh Davson 



significantly higher than that of the extracellular fluid. This 

 is illustrated by Fig. 1, and it follows that an equilibrium will 

 only be achieved when a counter-pressure is exerted on the 

 plasma equal to the colloid osmotic pressure due to the plasma 

 proteins. The amount of this difference of osmotic pressure is 

 determined by the concentration and degree of dissociation 

 of the proteins. Because of the high molecular weights of the 

 plasma proteins, their concentration, expressed as moles per 

 litre, is small and the difference of osmotic pressure that must 

 be resisted, if the system is to remain stable, is correspond- 

 ingly small, namely 25 mm. Hg. As a result, the organism is 

 able to maintain a statistical equilibrium between plasma and 

 extracellular fluid by virtue of the capillary pressure; at the 



Plasma Membrane Extracellular 



Fluid 



Na+ P~ 

 Na+ CI- 



Na+ CI- 



Fig. 1. The plasma-extracellular fluid 



system. 



(P=protein). 



arterial end of the capillary the pressure is greater than this 

 difference of osmotic pressure so that fluid flows into the 

 extracellular compartment; at the venous end the reverse 

 holds, and fluid is absorbed. 



It is worth noting that by the term "impermeability" 

 to a solute — here the plasma proteins — we do not necessarily 

 mean an absolute barrier; this is an ideal case on which cal- 

 culations are based, but practically it seems unlikely that a 

 natural membrane is completely impermeable to any of the 

 naturally occurring molecules in solution in the fluids, and it is 

 sufficient for our purposes if by "impermeability" is meant 

 that the rate of transport of this solute across the membrane 

 is negligibly small compared with that of the other molecules 

 that we are considering — in the particular case of plasma and 

 exti-acellular fluid, the salts and water. 



The cell membrane is a more selective barrier than the 



